The present invention relates to an industrial robot. More particularly, the invention relates to an apparatus for controlling the movements and work positions of an industrial robot.
Industrial robots are finding a wide range of application, not only for dangerous tasks, but also for routine operations. For example, assembly robots are currently being used on production lines for supplying, discharging and machining parts, operations which require highly precise positioning. The required motions are repeatedly effected by a robot control apparatus with a predetermined, designated point serving as a reference position.
Production lines in which industrial robots are used employ a transfer device with which products (workpieces) are moved along transfer lines or free flow lines. Pallets, for instance, may be used for the transfer device. If such pallets are positionally displaced from the reference positions in the production line, robot operations may become erroneous, even if the robot performs the designated motions with high precision. Therefore, most production lines incorporating industrial robots have been limited to situations in which the pallets can be positioned with a very high accuracy. In other words, most known types of industrial robot cannot be incorporated into existing production lines. If known robots were, on the other hand, used in such production lines, the type of operation done by the robots was restricted due to the low accuracy of the transfer device.
U.S. Pat. No. Re. 30,016 discloses a method and apparatus for operating an industrial robot in which command signals representing the position of a machine element relative to a workpiece are varied as a function of measured unprogrammed changes in the relative positions of the machine element and the workpiece. A first signal is produced which is representative of the direction and magnitude of a change in relative position between the machine element and the workpiece. A second signal is produced which represents an estimate of an unprogrammed change in relative position during a queuing delay related to a number of interrupt signals. These two signals are used to modify the command signals which operate the machine element to cause the machine element to move to the correct position relative to the workpiece.
United Kingdom patent application No. 2,027,938 A describes an industrial robot system which is an improvement over that described in the United States Reissue Patent. In the system of the United Kingdom Patent Application, a control memory stores data representing a precomputed optimum path of movement of the manipulator arm for a fixed position of a workpiece. Projected workpiece displacements are also stored therein. During playback operations, the movement of the workpiece is detected and an optimum path computed from the precomputed fixed path solution and data related to the current position of the workpiece.
Neither of those two references, however, describes how the position of a workpiece is to be measured or precisely how the position correction operation is performed.
It is thus an object of the present invention to provide a robot control apparatus for enabling a robot to move stably as desired without being affected by the accuracy with which pellets can be positioned.